Rigid closed cell thermoplastic foams have been used extensively as thermal insulating materials because of light weight, good compressive strength and high insulating values. However, their rigidity and inelasticity are adverse factors for application to curved surfaces such as pipe lines and cylindrical or spherical tanks. Cutting pieces to fit or custom molding incur added fabrication problems and costs. Yet, if such foams are forceably applied to a curved surface, the closed cell structure is often cracked or broken resulting in loss of insulation value.
Alternately, Nakamura U.S. Pat. No. 3,159,700 describes a process for directional flexibilization of rigid plastic foam sheets by partial compression or crushing an expanded foam sheet in a direction generally normal to that of desired flexibility. The process is designed to introduce wrinkles into the cell wall of the plastic foam without rupturing the foam cells or causing significant loss of compressive strength in other directions. By repeating the process in a direction substantially at right angles to the first, two-directional flexibilization can be achieved giving a foam product which can assume to a limited degree a compound curvature.
Such properties are particularly valuable for rigid foam sheet to be used for low temperature insulation of pipelines, tanks, and other large vessels for the transportation and storage of low temperature fluids. Furthermore, such flexibilized pieces or sheets of expanded foam are readily assembled by the spiral generation techniques of Wright U.S. Pat. No. 3,206,899 and Smith U.S. Pat. No. 4,017,346.
However, insulating requirements for the transportation and storage of liquid petroleum gas (LPG) and cryogenic fluids such as liquid nitrogen demand even higher long term resistance to water vapor transmission while retaining compressive strength adequate for field application and use. Cell wall cracking and rupture must be reduced to a minimum.
Accordingly, the present invention has for its objects providing a synthetic resin foam which:
(1) can be easily applied to a curved surface and then heated to secure the bent shape;
(2) has improved flexural workability and resistance to cracking, breaking or tearing;
(3) maintains effective, long term compressive strength and insulating properties necessary for low temperature storage and transport of liquefied natural gas and cryogenic fluids; and
(4) has high creep resistance and lasting crack resistance in biaxial directions essential to tolerate heavy loads under cryogenic storage conditions.